1. Field of the Invention
The present invention relates to an image pickup device adapted to expand the dynamic range of photometry, an image pickup apparatus including the image pickup device, a method of controlling the image pickup apparatus, and a program for implementing the control method.
2. Description of the Related Art
Today, to perform photometry for controlling exposure, many image pickup apparatuses employ either a method using a compression sensor for use in photometry or a method using a linear sensor, such as an image pickup surface. The method using a linear sensor has a smaller measurable dynamic range than the measurable dynamic range provided by the method using a compression sensor, and hence cannot accurately photometer an object of high contrast.
Particularly in a case where strobe light control is performed prior to strobe shooting by measuring the amount of reflected light reflected from an object by preliminary light emission and calculating the amount of light to be emitted for the shooting based on the measured reflected light amount, the amount of reflected light differs largely depending on a shooting situation. For this reason, the result of exposure at the preliminary light emission may not fall in the dynamic range of the linear sensor, causing a blackout condition or saturation. In such a case, it is impossible to perform accurate photometry by one-time exposure.
To cope with the above-mentioned problem, there has been proposed the following technique (see e.g. Japanese Laid-Open Patent Publication (Kokai) No. 2000-187266), in which it is determined, based on the result of photometry performed by preliminary light emission, whether or not preliminary light emission is required to be performed once again, and if it is determined that preliminary light emission is required to be performed again, photometry is carried out again by preliminary light emission after changing exposure conditions.
Further, a technique for expanding the dynamic range of the linear sensor has been proposed the following technique (see e.g. Japanese Laid-Open Patent Publication (Kokai) No. 2005-117192), in which using a solid-state image pickup apparatus, a plurality of kinds of summations of pixel signals of the same color are performed to generate signals different in the number of added pixel signals, and photometry is carried out based on the output signals.
However, in the technique disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2000-187266, it is required to carry out preliminary light emission for photometry a plurality of times, which increases a release time lag between a time point a release switch operates upon depression of a shutter button of the image pickup apparatus and a time point shooting starts to be executed.
Further, the technique disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2005-117192 is not very compatible with a live view function (i.e. a function of checking a composition and focusing while displaying an image formed on an image pickup device in real time) which is realized using a CMOS as the image pickup device. More specifically, the pixel summations cause degraded resolution, which results in lowered photometric accuracy.
Differently from a CCD image pickup device, the CMOS image pickup device is not capable of transferring electric charges in a single operation, and hence pixel signals are sequentially read out. For this reason, in a situation where a mechanical shutter cannot be used as in the case where live view is being performed, exposure is continued even during pixel signal read-out operation. This makes it necessary, when luminance is high, to speed up a read-out operation for reading out pixel signals from the image pickup device by thinning-out reading to prevent normal light from causing saturation of pixel signals, thereby reducing accumulation time.
However, when the pixel summation method proposed in Japanese Laid-Open Patent Publication (Kokai) No. 2005-117192 is employed to expand the dynamic range using the CMOS as the image pickup device, not only thinning-out reading of pixel signals but also the pixel summation causes degraded resolution. This causes the problem of lowered photometric accuracy. If it is possible to secure a resolution high enough not to affect photometric accuracy, it is desired to further increase a thinning rate to achieve high-speed read-out of pixel signals so as to expand the photometric dynamic range toward a high luminance side.